Updating to automatically dismiss onBeforeUnload event dialogs in IE

This is a change from previous behavior, where such dialogs were
handled via the standard alert handling commands. The W3C WebDriver
Specification demands that onBeforeUnload dialogs be dismissed
automatically, with no input allowed for the user. This may be a
breaking change for some users who are currently relying on the
alert-handling commands to handle these alerts; however, not
implementing this behavior makes the driver not compliant with the
specificiation. Users in that state will have no choice but to
modify their code to accomodate the new, spec-compliant behavior.